The present invention is directed to a leak detector system for connection into a pressurized liquid flow system which is operated intermittently, and a leak detector system which is specially constructed to monitor the flow piping system for leakage upon each actuation and demand for delivery.
Retail gasoline dispensing stations include a central storage tank to which a plurality of gasoline dispenser are connected. The tank is provided with a single submersible motor-pump unit connected to a common discharge header and distribution pipe for delivery of gasoline to each of the dispensers which is connected to the distribution pipe by a branch pipe. Each dispenser is provided with a control valves and switching means for withdrawal of gasoline. Generally, the system includes a mechanically actuated switch which is manually operated by the purchaser or service station attendant. Actuation of the switch energizes the solenoid valve to open the line to the nozzle and energizes the submersible motor-pump to provide for delivery of gasoline from the dispenser. A plurality of dispensers can of course be simultaneously withdrawing gasoline from the common storage tank. Each pump is provided with the same similar controls. Generally, in such systems care must be taken to prevent leakage of gasoline from the distribution system. Thus, any leak in the system creates an extremely hazardous condition. Leakage is particularly great under pumping conditions when the pumps are pressurized. As the result code requirements generally include some requirement for a leak detection means incorporated into the flow system.
A well known and widely used detection system includes a pressure responsive detector connected to the immediate discharge side or discharge connection of the submersible pump or storage tank. The leak detector includes a pressure responsive control which compares the pump pressure with the line pressure. If the pressure in the distribution line does not rapidly rise to the output pressure of the pump, the leak detector automatically responds to prevent normal discharge. Generally, the leak detector operates to completely close the line to prevent withdrawl of gasoline or otherwise significantly reduces the flow. Otherwise, the leak detector unit permits the continued full flow and normal pressurized discharge. Although such systems are widely used and well known, it is also known that false actuation under apparent normal discharge operation may occur. Thus, in addition to actual leakage, other conditions may arise which simulate a leakage condition even though in fact a non-hazardous condition exists. For example, if the operator actuates the nozzle switch and rapidly opens the nozzle, a decrease in the pressure in the distribution line is created. The pressure responsive leak detector requires a certain time in which to operate. The rapid opening of the nozzle may result in the reduced pressure and thus in a condition simulating a leak prior to the completion of the leak detector cycle. The leak detector of course responds to maintain the fully closed or restricted flow condition. The operator must then completely recycle the system operation and, if he does not take care, may again create a simulated leakage condition. Such problems are particularly encountered during the winter months and particularly in geographical areas in which the air temperature is low while the ground temperature is relatively high. Thus, in the northern hemisphere it is not unusual to have the buried storage tank at ground temperature of approximately fifty degrees Fahrenheit. The distribution line is relative close to upper ground level, and the gasoline therein closely approximates the air temperature, which may well be zero degrees Fahrenheit or even colder. Under such a condition, the gasoline dispensed is at the fifty degree Fahrenheit. When the pump is shut down and the system turned off, the distribution line of course is filled with the higher temperature gasoline. Over a relatively short period of time, the gasoline cools and contracts due to the reduction in temperature from fifty degrees Fahrenheit to that of zero degrees Fahrenheit or essentially ground level temperature. Upon the next actuation of any dispensers on the distribution, the pump starts. However, the output of the pump must first fill the distribution pipe. During this period of course, the leak detector unit is monitoring the pressure deifferential and will continue to do so until the pump establishes full line pressure in the line. If the nozzle valve is opened prior to pressurization of the line, a leakage signal to the leak detector will be created. Other circumstances may also readily occur. Thus, if one introduces the nozzle into the receiving tank and opens the nozzle simultaneously with another person turning on the dispenser switch, the line will be opened and the leak detector will immediately be presented with the low pressure condition. In fact it has been found that the majority of, if not substantially all, complaints of such "nuisance tripping" is attributable to a too rapid opening of the discharge nozzle at the dispenser. Although in a full service station operation, the attendants might well be educated to properly operate the dispenser with the proper sequence, accidental malfunctions may still occur. Of course, in a self-service station such education is extremely difficult, if not impossible, and the nuisance tripping may be frequently encountered.
In retail gasoline dispensing systems, the dispensing hose may also have been drained by the user after the system has been shut down. Upon start up, the pump unit must again fill the hose system during the cycle time. Although this situation alone may not cause a false signal, the sequence does insert an additional delay in the build-up of pressure within the flow system, and may with other factors result in erroneous signaling of the leak.
Another difficulty which is encountered is that the various dispensers have different reset times. For example, certain pumps may have computer reset time which is longer than that normally encountered. This additional time is such that the nozzle may be opened before the proper time to allow the leak detector unit to cycle and once again giving a false indication.
Recognization of this problem has resulted in various proposed solutions including sensing means for monitoring of the leak detector operation on shut down of a pumping cycle. For example, U.S. Pat. No. 3,935,567 discloses a leak detection including a pressure responsive leak detector connected into the distribution line and having means for measuring the bleed-down rate of the system after a discharge cycle is completed. Thus, pumping pressure is normally on the order of twenty pounds per square inch, which is normally maintained for a period of time after closing of the nozzle(s). Any leakage in the system which is present will quite rapidly bleed off such pressure. As suggested in this U.S. Pat. No. 3,935,567, a pressure sensitive switch may be coupled to the circuit and provides an indication of either a leak or nonleak condition by monitoring the bleed off time of the line. A similar bleed down-time of a pressure responsive switch valve is suggested in U.S. Pat. No. 3,738,531 which issued June 12, 1973. In this system, the fluid pressure responsive switch valve in-line prevents the discharge until the fluid pressure in the line reaches a selected level. Alternatively, the inventor has suggested the use of a mechanically activated switch coupled to the leak detector valve. Thus, as in U.S. Pat. No. 3,935,567 the U.S. Pat. No. 3,738,531 suggests a control operative on shutdown in response to the position of the leak detector valve.
Other patents which similarly show leak detection on shut-down are shown in U.S. Pat. No. 3,188,127 which issued Jan. 29, 1974 and is assigned to Dresser Industries, Inc., as well as the related U.S. Pat. No. 4,131,216.
Although such leak detector apparatus is widely used and various leak detection monitor means have been suggested, the prior art generally requires rather complex control and pressure sensing systems and the like interconnected into the flow system. Therefore, a need exists for a simple but reliable leak detection monitor means which can be used with all of the several types of leak detector units and pumping systems including those requiring retrofitting.